Singlet oxygen

Singlet oxygen is the common name used for the two metastable states of molecular oxygen (O₂) with higher energy than the ground state triplet oxygen ^[1]. The energy difference between the lowest energy of O₂ in the singlet state and the lowest energy in the triplet state is about 3625 kelvin (T_e (a¹Δ_g <- X³Σ_g^-) = 7918.1 cm^-1.)

Molecular oxygen differs from most molecules in having an open-shell triplet ground state, O₂(X³Σ_g^-). Molecular orbital theory predicts two low-lying excited singlet states O₂(a¹Δ_g) and O₂(b¹Σ_g⁺) (for nomenclature see article on Molecular term symbol). These electronic states differ only in the spin and the occupancy of oxygen's two degenerate antibonding π_g-orbitals (see degenerate energy level). The O₂(b¹Σ_g⁺)-state is very short lived and relaxes quickly to the lowest lying excited state, O₂(a¹Δ_g). Thus, the O₂(a¹Δ_g)-state is commonly referred to as singlet oxygen.

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Physics

The energy difference between ground state and singlet oxygen is 94.2 kJ/mol and corresponds to a transition in the near-infrared at ~1270 nm. In the isolated molecule, the transition is strictly forbidden by spin, symmetry and parity selection rules, making it one of nature's most forbidden transitions. In other words, direct excitation of ground state oxygen by light to form singlet oxygen is very improbable. As a consequence, singlet oxygen in the gas phase is extremely long lived (72 minutes). Interaction with solvents, however, reduces the lifetime to microsecond or even nanoseconds.

Direct detection of singlet oxygen is possible through its extremely weak phosphorescence at 1270 nm, which is not visible to the eye. However, at high singlet oxygen concentrations, the fluorescence of the so-called singlet oxygen dimol (simultaneous emission from two singlet oxygen molecules upon collision) can be observed as a red glow at 634 nm ^[2].

Chemistry

The chemistry of singlet oxygen is different from that of ground state oxygen. Singlet oxygen can participate in Diels-Alder reactions and ene reactions. It can be generated in a photosensitized process by energy transfer from dye molecules such as rose bengal, methylene blue or porphyrins, or by chemical processes such as spontaneous decomposition of hydrogen trioxide in water or the reaction of hydrogen peroxide with hypochlorite ^[3]. Singlet oxygen reacts with an alkene -C=C-CH- by abstraction of the allylic proton in an ene reaction type reaction to the allyl hydroperoxide HO-O-C-C=C. It can then be reduced to the allyl alcohol. With some substrates dioxetanes are formed and cyclic dienes such as 1,3-Cyclohexadiene form [4+2]cycloaddition adducts. ^[4].

Biochemistry

In photosynthesis, singlet oxygen can be produced from the light-harvesting chlorophyll molecules. One of the roles of carotenoids in photosynthetic systems is to prevent damage caused by produced singlet oxygen by either removing excess light energy from chlorophyll molecules or quenching the singlet oxygen molecules directly.

In mammalian biology, singlet oxygen is a form of reactive oxygen species, which is linked to oxidation of LDL cholesterol and resultant cardiovascular effects. Polyphenol antioxidants can scavenge and reduce concentrations of reactive oxygen species and may prevent such deleterious oxidative effects ^[5].

Singlet oxygen is the active species photodynamic therapy.

References